Some day I was stucked thinking about some new design flaws, vaping a little and making some O-rings and tricks waiting for the inspiration to come. Suddenly I noticed that would be cool doing more than vapor O-Rings and wondered if I could make X-Rings, then this design came to my mind. Just put the main tube in your mouth and make little puffs usual, it will emit vapor X's. Cool like magic. To remove or change the cap simply press it out with a pencil or something alike. Also after playing with this and make some XRings I found the solution to the design flaw I was thinking about initially.

Two part print. The main tube that can be used alone to make perfect giant O-Rings, or press fitting the X-Cap to make vapor X-Rings. Easy print, no support required.

Other thoughts: I am wondering if it could be done some design to make vapor ships like Gandalf does... Maybe I add some other cap variants in the future hehe...

UPDATE 08-JUN-2017: V2. Enhanced design to cilyndric fit. I don't know why I did it conical!! DOH! XRing redesigned air flux (V1 didn't work at all, maybe this is not possible at this size), increased diameter for the 5-O-Ring for the same reason.